Subject: [SLR-Mail] No. 1123: Meteorological Devices Best Practices
From: Van S Husson, ILRS CB <Husson, Van <Van.Husson@honeywell-tsi.com>>

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SLR Electronic Mail          2003-08-28 18:49:00 UTC           Message No. 1123 
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Author: Van S Husson, ILRS CB
Subject: Meteorological Devices Best Practices

Dear Colleagues: 

  Meteorological errors from our sites have been a common 
  problem in our technique, which adversely influences final 
  SLR data quality. As we push towards 1 millimeter (mm)
  accuracy, we need to eradicate meteorological errors as
  an error source.
 
  An error of just 1 millibar INDUCES an elevation dependent
  range bias of 7 and 3 mm at 20 and 90 degrees elevation,
  respectively. We also can not neglect the humidity
  measurement.
 
  A 10 % humidity error, at a temperature of 20 degrees 
  Celsius, INDUCES an elevation dependent range bias of 1 
  and 0.4 millimeters at 20 and 90 degrees, respectively. 
  The impact of a humidity error is temperature dependent,
  the higher the temperature the larger the error.

  The temperature measurement is not critical for 
  tropospheric refraction, but is a critical measurement 
  for systems which have far calibration targets
  (i.e. calibration distance >1 Km). 
 
  Below is a list of recommended procedures and manufacturers:
  
  BEST PRACTICES
  --------------
  Each site should have a least one near-by independent
  meteorological data source (i.e. local airport, weather
  station, etc.) for comparison purposes. We recommend that
  met. comparisons been done automatically at least 
  daily and tabulated (i.e. create a database) in order to 
  detect any long term trend in the differences. 
 
  At least ONCE EVERY 2 years, we strongly encourage
  that your met. device be calibrated versus a known standard.
  This is currently being done within the NASA network and
  a similar campaign was initiated in 2002 within EUROLAS.

  Through regular calibrations and daily comparisons any errors
  that might creep into the measurements would be traceable and
  therefore recoverable. Any met. data correction algorithms
  should be sent to the ILRS Central Bureau (cb@ilrs.gsfc.nasa.gov)
  and footnoted in your site log. 

  It is also imperative that your barometric sensor either
  be at the same height as your optical axes OR that the 
  pressure measurement be corrected for height in on-site data
  processing. A one meter height difference equates to an 
  approximate 0.1 millibar pressure change.

  Meteorological devices with ports/gills are preferable to 
  devices with tubing. Tubes can become easily kinked 
  or clogged. Tubing should also be shielded from the wind.
 
 
  MANUFACTURERS
  -------------
  Based on the site logs, the two most popular manufacturers
  of meteorological sensors are Paroscientific (16 sites) and
  Vaisala (6 sites). The Paroscientific MET3 is the most
  widely used model within the ILRS. Below are URLs of these
  two manufacturers:

    Vaisala Web Site is:
    http://www.vaisala.com/

    Paroscientific Web Site is:
    http://www.paroscientific.com/
  
    Paroscientific MET3A description is located at:
    http://www.paroscientific.com/met3a.htm 

  
  The key features that should be strongly considered in the purchase 
  of any met. device for our application is:
   
    1. Automated readings,

    2. the long term stability and absolute accuracy of the 
       barometric sensor, and

    3. rapid humidity recovery time after saturation. 


  The purchase and proper installation/care of a state-of-the-art
  meteorological device coupled with periodic calibrations 
  should significantly improve our data quality. The cost of a good
  met. device is a relatively small price to pay factoring
  in the total price of an SLR system especially and its potential 
  benefits.

From: &#8221;Husson, Van&#8221; <Van.Husson@honeywell-tsi.com> 

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